Lahar Hazards at Three Sisters

Explosive eruptions or large debris avalanches on the
Three
Sisters can generate lahars of sufficient volume to travel tens of kilometers from source areas. The
McKenzie and Deschutes River, as well as their tributaries that head at Three Sisters, may be affected,
which could send lahars into communities such as McKenzie Bridge or Sisters or even those farther
downstream. Small lahars were generated by rhyolite eruptions at South Sister about 2 ka,
though
much larger lahars swept down valleys during earlier eruptions. Eruptions that deposit large amounts
of
sediment in watersheds are typically followed by years to decades of unstable channels and increased
flooding, even in areas beyond the reach of direct effects of the eruption. During the past century, at
least
five small lahars were generated by failure of glacial-moraine dams that impounded small lakes on
all of
the Three Sisters and Broken Top. Serious effects of these events were largely restricted to
undeveloped
areas within 10 km (6 mi) of their source.